'Super-terminal' raindrops break speed limit

Many raindrops travel at "super-terminal" velocities, faster than was thought possible. As a result, meteorologists may be miscalculating how much it rains.

Previously, it was assumed that all raindrops fall at terminal velocity, a constant maximum speed that is determined by the interplay of gravity and drag. The velocity for individual drops is considered to be largely controlled by their size: larger drops fall faster than smaller drops, due to their greater mass.

Fernando García-García of the National Autonomous University of Mexico and colleagues measured the shadows of natural raindrops passing through a ray of infrared light. They found that up to half exceed their terminal velocity. Some travel as much as 10 times faster, for their size.

Broken drops

"Others had detected this before, but everybody disregarded it, blaming it on an error," says Garcia-Garcia.

The team suspects that the super-terminal drops may be fragments of larger drops broken apart as they fall. "If a large drop breaks into several fragments, each drop will have the speed of the large drop, at least temporarily, until the smaller drops slow to their new terminal velocity," García-García says.

As a result, meteorologists may be overestimating total rainfall by up to 20 per cent, say the team. Weather forecasters use total rainfall figures to predict floods, and climatologists use the estimates to gauge how rain patterns are changing with climate change.

Rain radar

Total rainfall over an area is generally calculated indirectly, by using a Doppler radar that bounces electromagnetic waves off the raindrops. The radar can measure rainfall over thousands of square kilometres.

But the radar records only the speed of drops. This is then used to infer the average size of the drops. This is then multiplied by the number of drops to give a total volume of water.

"If you get a signal back and see drops are moving quickly, you assume they are big drops, but in reality they are small drops moving faster," says Garcia-Garcia.

Ana Barros of Duke University in Durham, North Carolina, says meteorologists' rainfall estimates are unlikely to be too far wrong, pointing out that the super-terminal drops would soon slow down after fragmenting from larger drops.

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